The field of the present invention is radially outward flow turbines.
Two-phase turbines have long been contemplated as needed in a wide variety of applications, both terrestrial and space. The ability to handle two-phase flow would allow such devices to be coupled directly to a source of pressurized fluid containing or flashing to contain both a liquid phase and a gas phase. Thus, the primary fluid in a number of applications would constitute the working fluid for the turbine. Examples of such fluids are geothermal brine and hydrocarbons with dissolved gas. Presently, without the availability of an effective two-phase turbine, such fluids are simply throttled with a total waste of extractable mechanical energy. It is contemplated that practical two-phase turbines would find substantial utility in open or closed energy conversion cycles. They could be used as a key element in topping or bottoming cycles. There are also many possible special applications of two-phase turbines as well. Examples include flashing of geothermal brine and boiler blowdowns.
The theoretical thermal efficiencies of two fluid component, two-phase systems approach the Carnot cycle efficiencies due to reheat of vapor by the liquid phase during expansion. This potentially means that considerably more mechanical or electrical energy can be generated from the same thermal energy from a heat source. Even for single component, two-phase turbine systems, the thermal efficiencies are higher than for comparable conventional Brayton or Rankine cycles.
In order to realize the full energy conversion potential of a two-phase turbine, it is necessary to design such a turbine having high turbine efficiency. In most of the development work on two-phase turbines in the past, substantial mechanical flow energy losses were experienced within such two-phase turbines. Demonstrated turbine efficiencies were at best in the range of 40%-45% for long-lasting devices. In the case of two-phase flow impinging on blades of a conventional axial flow steam turbine, efficiency of up to 52.4% was measured. However, blade erosion was prohibitive, severely limiting turbine life.
Extensive experimental investigation of the performance of a two-phase reaction turbine (Hero type turbine) as shown in FIG. 1 has been conducted. Saturated hot water was introduced into the rotor through a hollow shaft 2. The saturated water then flowed radially outwardly through passages 3 and was flashed through straight, short DeLaval nozzles tangentially to the rotor. The DeLaval nozzles consisted of two parts, a very short liquid converging nozzle 4 and a longer straight diverging two-phase nozzle 5. At the end of the liquid nozzle a discontinuous enlargement 6 of two to three times the cross-sectional area of the nozzle was used. At a top speed of 8066 RPM, the best measured turbine efficiency was 33% with this device. Calculations suggest that the efficiency of the DeLaval nozzles of this device were only 46%. If the nozzle efficiency was 80%, and 70% of the existing kinetic energy was recovered, then the turbine efficiency would have been 75%. If such efficiencies could be achieved, two-phase turbines would become very competitive for many applications.
In rotating stepped DeLaval nozzles such as illustrated in FIG. 1, severely delayed flashing has been found to occur. That is, as the saturated liquid travels from the center toward the outer radial edge of the rotor it becomes highly subcooled due to a very large pressure increase (about 100 atmospheres), caused by the very substantial centrifugal accelerations of the order of several thousand g's. Flashing of highly subcooled liquid through very short nozzles yields low efficiency in such cases. Vapor nucleation sites are too small and do not have enough time to grow while the liquid passes quickly through the short converging part of the nozzle.
Another inefficiency of such devices as illustrated in FIG. 1 is the existence of large lateral accelerations (on the order of multiple thousands of g's). This extreme lateral acceleration tends to separate the two phases and drastically increase the velocity slip loss. The slip loss is the loss of nozzle efficiency due to the vapor moving faster than the liquid.
U.S. Pat. No. 4,332,520 to House discloses a two-phase reaction turbine. The features of this turbine fail to address the major problems of lateral phase separation and abrupt flashing. The patent teaches a decrease of kinetic energy losses of the discharge. This is achieved by decreasing rotational speed of the reaction turbine rotor. The rotor, acting through a mechanical gear, drives a liquid pump which is located coaxially with the reaction turbine rotor. A pump acts to increase velocity of the liquid before it is injected into the reaction turbine rotor, presumably to decrease the inlet losses.
U.S. Pat. No. 4,336,039 to Sohre discloses a turbine into which two-phase, single component saturated fluid (such as geothermal brine) is introduced. The turbine rotor first separates the two phases by means of centrifugal forces generated through induced rotational motion of the fluid. Subsequently, saturated liquid and steam are expanded through separate reaction nozzles carried by the same single turbine rotor. Separation and throttling of two-phase fluid is common practice in geothermal power plants but extraction of mechanical energy from saturated flashing liquid is not. Recovery of mechanical energy from saturated flashing liquid causes severe erosion of conventional turbine blades. Also, such devices have very poor efficiency.
German Patent No. 213,973 to Jonas discloses a two-phase flow reaction turbine much like that illustrated in FIG. 1. A very short nozzle at the rotor outer radius is also disclosed in this reference and the design is believed to be of low efficiency.